1. Field of the Invention
The present invention relates to a process for producing vitamin A acid from vitamin A aldehyde.
2. Description of the Related Art
Vitamin A acid is known to possess the function of curing various diseases and lesions, for example those caused by deficiency of vitamin A. Thus, there has been suggested possibility of vitamin A acid being usable as a therapeutic agent for deficiency of vitamin A and like symptoms.
Many processes are known to produce vitamin A acid (see for example "The Retinoides, Volume 1, Michael B. S. et al., Academic Press"). Among those processes, one that performs the production in the shortest procedure comprises using, as starting material, vitamin A acetate, which is being produced on an industrial scale in a large amount and is readily available, converting it via vitamin A to vitamin A aldehyde and then oxidizing the aldehyde. Vitamin A can readily be obtained by hydrolyzing vitamin A acetate under the usual alkaline condition. A known process for producing vitamin A aldehyde from vitamin A is the Oppenauer oxidation using tertiary aldehyde (Japanese Patent Application Laid-open No. 206076/1991). A known process for converting vitamin A aldehyde or its analogues to vitamin A acid structure comprises oxidizing the former with a catalyst of manganese oxide, manganese oxide and sodium cyanide, silver oxide or the like (see for example Kaneko, R. et al., Chem. Ind. (London) 1971, (36), 1016, and Barua, R. K., Curr. Sci. 37, 364 (1968)). This process for producing vitamin A acid however uses an expensive and toxic heavy metal compound such as manganese or silver and is hence rarely employed on an industrial scale in practice.